In a wireless communication system, a base station may provide one or more coverage areas, such as cells or sectors, in which the base station may serve user equipment devices (UEs), such as cell phones, wirelessly-equipped personal computers or tablets, tracking devices, embedded wireless communication modules, or other devices equipped with wireless communication functionality (whether or not operated by a human user). In general, each coverage area may operate on one or more carriers each defining a respective bandwidth of coverage, and each coverage area may define an air interface providing a downlink for carrying communications from the base station to UEs and an uplink for carrying communications from UEs to the base station. The downlink and uplink may operate on separate carriers or may be time division multiplexed over the same carrier(s). Further, the air interface may define various channels for carrying communications between the base station and UEs. For instance, the air interface may define one or more downlink traffic channels and downlink control channels, and one or more uplink traffic channels and uplink control channels.
In accordance with the Long Term Evolution (LTE) standard of the Universal Mobile Telecommunications System (UMTS), for instance, each coverage area of a base station may operate on one or more carriers spanning 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz. On each such carrier used for downlink communications, the air interface then defines a Physical Downlink Shared Channel (PDSCH) as a primary channel for carrying data from the base station to UEs, and a Physical Downlink Control Channel (PDCCH) for carrying control signaling from the base station to UEs. Further, on each such carrier used for uplink communications, the air interface defines a Physical Uplink Shared Channel (PUSCH) as a primary channel for carrying data from UEs to the base station, and a Physical Uplink Control Channel (PUCCH) for carrying control signaling from UEs to the base station.
In LTE, downlink air interface resources are mapped in the time domain and in the frequency domain. In the time domain, LTE defines a continuum of 10-millisecond (ms) frames, divided into 1 ms subframes and 0.5 ms slots. With this arrangement, each subframe is considered to be a transmission time interval (TTI). Each frame has 10 subframes, and each subframe has 2 slots. In the frequency domain, resources are divided into groups of 12 sub-carriers. Each sub-carrier is 15 kHz wide, so each group of 12 sub-carriers occupies a 180 kHz bandwidth. The 12 sub-carriers in a group are modulated together, using orthogonal frequency division multiplexing (OFDM), in one OFDM symbol.
LTE further defines a particular grouping of time-domain and frequency-domain resources as a downlink resource block. In the time domain, each downlink resource block has a duration corresponding to one sub-frame (1 ms). In the frequency domain, each downlink resource block consists of a group of 12 sub-carriers that are used together to form OFDM symbols. Typically, the 1 ms duration of a downlink resource block accommodates 14 OFDM symbols, each spanning 66.7 microseconds, with a 4.69 microsecond guard band (cyclic prefix) added to help avoid inter-symbol interference. Depending on the bandwidth of the downlink carrier, the air interface may support transmission on a number of such downlink resource blocks in each subframe. For instance, a 5 MHz carrier supports 25 resource blocks in each subframe, whereas a 15 MHz carrier supports 75 resource blocks in each subframe.
The smallest unit of downlink resources is the resource element. Each resource element corresponds to one sub-carrier and one OFDM symbol. Thus, a resource block that consists of 12 sub-carriers and 14 OFDM symbols has 168 resource elements. Further, each OFDM symbol and thus each resource element can represent a number of bits, with the number of bits depending on how the data is modulated. For instance, with Quadrature Phase Shift Keying (QPSK) modulation, each modulation symbol may represent 2 bits; with 16 Quadrature Amplitude Modulation (16QAM), each modulation symbol may represent 4 bits; and with 64QAM, each modulation symbol may represent 6 bits.
Within a resource block, and cooperatively across all of the resource blocks of the carrier bandwidth, different resource elements can have different functions. In particular, a certain number of the resource elements (e.g., 8 resource elements distributed throughout the resource block) may be reserved for reference signals used for channel estimation. In addition, a certain number of the resource elements (e.g., resource elements in the first one, two, or three OFDM symbols) may be reserved for the PDCCH and other control channels (e.g., a physical hybrid automatic repeat request channel (PHICH)), and most of the remaining resource elements (e.g., most of the resource elements in the remaining OFDM symbols) would be left to define the PDSCH.
Across the carrier bandwidth, each subframe of the LTE air interface thus defines a control channel space that generally occupies a certain number of 66.7 microsecond symbol time segments (e.g., the first one, two, or three such symbol time segments), and a PDSCH space that generally occupies the remaining symbol time segments, with certain exceptions for particular resource elements. The actual number of symbol segments that the control channel space occupies in a given subframe is indicated by a Control Format Indicator (CFI) carried by a Physical Control Format Indicator Channel (PCFICH). In each subframe, the PCFICH is defined in 16 resource elements of the first symbol time segment. The 16 resource elements are spread across four resource element groups (REGs), with each REG equidistantly spread apart from each other and containing four consecutive resource elements (or four consecutive resource elements separated by one or more reference signals). The exact location of the four REGs within the first symbol time segment is determined using a formula having inputs that include a physical cell ID (a number assigned to each base station, ranging from 0 to 503) and carrier bandwidth.
Further, the 16 resource elements of the PCFICH carry 16 modulation symbols. With QPSK modulation, each modulation symbol represents two bits, such that the 16 resource elements carry 32 bits of data. These 32 bits of data cooperatively form a binary codeword corresponding to a CFI. Depending on the configuration of the control channel space for a given subframe, the binary codeword may be one of three binary codewords that each correspond to a respective CFI and respective number of symbol time segments. In practice, the UE may determine the CFI carried by the PCFICH in each subframe to determine the format of the control channel space of the subframe. For instance, the UE may determine that the PCFICH of a given subframe carries a first binary codeword corresponding to a first CFI, meaning that the control channel space occupies the first symbol time segment of the given subframe, or determine that the PCFICH of the given subframe carries a second binary codeword corresponding to a second CFI, meaning that the control channel space occupies the first and second symbol time segments of the given subframe.
One of the main functions of the PDCCH in LTE is to carry “Downlink Control Information” (DCI) messages to served UEs. LTE defines various types or “formats of DCI messages, to be used for different purposes, such as to indicate how a UE should receive data in the PDSCH of the current subframe, or how the UE should transmit data on the PUSCH in an upcoming subframe. For instance, a DCI message in a particular subframe may schedule downlink communication of bearer data to a particular UE (i.e., a UE-specific data transmission), by specifying one or more particular PDSCH segments that carry the bearer data in the current subframe, what modulation scheme is used for that downlink transmission, and so forth. And as another example, a DCI message in a particular subframe may indicate the presence of one or more paging messages carried in particular PDSCH segments and may cause certain UEs to read the PDSCH in search of any relevant paging messages.
Each DCI message may span a particular set of resource elements on the PDCCH (e.g., one, two, four, or eight control channel elements (CCEs), each including 36 resource elements) and may include a cyclic redundancy check (CRC) that is masked (scrambled) with an identifier (e.g., a particular radio network temporary identifier (RNTI)). In practice, a UE may monitor the PDCCH in each subframe in search of a DCI message having one or more particular RNTIs. And if the UE finds such a DCI message, the UE may then read that DCI message and proceed as indicated. For instance, if the DCI message schedules downlink communication of bearer data to the UE in particular PDSCH segments of the current subframe, the UE may then read the indicated PDSCH segment(s) of the current subframe to receive that bearer data.